Cancer is a disease characterized by genomic instability. The acquisition of genomic instability is thought to arise from a coincident disruption of genomic integrity and a loss of cell cycle control mechanisms. Generally, a disruption of genomic integrity is thought merely to increase the probability that a cell will engage in the multistep pathway leading to cancer. However, coupled with a loss of cell cycle control mechanisms, a disruption in genomic integrity may be sufficient to generate a population of genomically unstable neoplastic cells. A common genetic change characteristic of transformation is loss of heterozygosity. Loss of heterozygosity at a number of tumor suppressor genes has been implicated in tumorigenesis. For example, loss of heterozygosity at the P53 tumor suppressor locus has been correlated with various types of cancer. Ridanpaa, et al., Path. Res. Pract, 191: 399-402 (1995). The loss of the apc and dcc tumor suppressor genes has also been associated with tumor development. Blum, Europ. J. Cancer, 31A: 1369-372 (1995).
Loss of heterozygosity is therefore a potentially useful marker for detecting the early stages of cancer. However, in the early stages of cancer only a small number of cells within a tissue have undergone transformation. Genetic changes characteristic of genomic instability theoretically can serve as markers for the early stages of, for example, colon cancer, and can be detected in DNA isolated from biopsied colonic epithelium and in some cases from transformed cells shed into fecal material. Sidransky, et al., Science, 256: 102-105 (1992).
Detection methods proposed in the art are time-consuming and expensive. Duffy, supra. Moreover, methods according to the art cannot be used to identify a loss of heterozygosity or microsatellite instability in small subpopulation of cells when the cells exist in a heterogeneous (i.e., clonally impure) sample. For example, in U.S. Pat. No. 5,527,676, it is stated that tissue samples in which a mutation is to be detected should be enriched for tumor cells in order to detect the loss of heterozygosity in a p53 gene.
Colorectal cancer is a common cause of death. Any tumor or precancerous polyp that develops along the length of the colon or the rectum sheds cells or DNA into the lumen of the colon. Shed cells or cellular DNA are usually incorporated onto and into stool as stool passes through the colon. In the early stages of cancer, cancerous or precancerous cells represent a very small fraction of the shed epithelial cells or DNA in stool. Current methods for early detection of colorectal cancer do not focus on detecting cancerous or precancerous cells in stool. Rather, such methods typically focus on extracellular indicia of the presence of cancer, such as the presence of fecal occult blood or carcinoembryonic antigen circulating in serum.
It is thought that sporadic colorectal cancers result from an accumulation of mutations in oncogenes and tumor suppressor genes. Sporadic colorectal cancer is also typically associated with massive loss of genetic material called loss of heterozygosity. Such mutations appear to occur at a point in the etiology of the disease that is much earlier than the point at which extracellular indicia or clinical signs of cancer are observed. If detected early, colon cancer may be effectively treated by surgical removal of the cancerous tissue. Surgical removal of early-stage colon cancer is usually successful because colon cancer begins in cells of the colonic epithelium and is isolated from the general circulation until the occurrence of invasion through the epithelial lining. Thus, detection of early mutations in colorectal cells would greatly increase survival rate.
Current non-invasive methods for detection of colon cancer involve the detection of fecal occult blood and carcinoembryonic antigen. These methods often either fail to detect colorectal cancer or they detect colorectal cancer only after it has progressed to a less treatable stage. Moreover, carcinoembryonic antigen is thought not to be an effective predictor of cancer but merely an indicator of recurrent cancer.
Invasive techniques, such as endoscopy, while effective, are expensive and painful and suffer from low patient compliance. Accordingly, current colon cancer screening methods are not practical for screening large segments of the population. See, e.g., Blum, Europ. J. Cancer, 31A: 1369-1372 (1995).
Therefore, there is a need in the art for simple and efficient non-invasive methods for reliable large-scale screening to identify individuals with early stage disease. Such methods are provided herein.